Currently, in the third generation partnership project (3GPP), an international standard “international mobile telecommunications (IMT)-2000” of a 3G mobile communication system designed by an international telecommunication union (ITU) is being standardized. Long term evolution (LTE) which is one of data communication specifications designed by the 3GPP is a long term advanced system aiming for the fourth generation (4G) IMT-Advanced, and is called “3.9G (super 3G).” One of features of 4G lies in that a maximum communication rate or a quality improvement at a cell edge can be implemented using a technique such as a relay or a carrier aggregation.
The LTE is a communication scheme based on an orthogonal frequency division multiplexing (OFDM) modulation scheme, and employs OFDMA as a downlink wireless access scheme. Here, the OFDM is a multi-carrier scheme that allocate a plurality of data to frequency sub carriers which are orthogonal, that is, do not interfere with each other, and inverse fast Fourier transform (IFFT) is performed on each sub carrier to transform each sub carrier at a frequency axis into a signal at a time axis, and then the transformed signal is transmitted. Since transmission data are distributed to a plurality of carriers whose frequencies are orthogonal to each other and transmitted, a band of each carrier is narrow, frequency utilization efficiency is very high, and it is robust to delay distortion (frequency selective fading interference) caused by multiple paths. Further, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which a single communication station does not occupy all sub carriers of OFDM signals, but a set of sub carriers on the frequency axis are allocated to a plurality of communication stations, and the sub carriers are shared among the plurality of communication stations. When a plurality of users use different sub carriers or different time slots (that is, division multiplexing is performed in the frequency direction and the time direction), communication can be performed without any interference.
Further, in the LTE, two ways of schemes of frequency division duplex (FDD) and time division duplex (TDD) can be selected.
In the FDD, an uplink-dedicated band and a downlink-dedicated band are used. In the uplink and the downlink, a format of a radio frame configured with 10 consecutive sub frames is used. Here, an uplink refers to communication from a user equipment (UE) (terminal) to an eNodeB (base station), and a downlink refers to communication from an eNodeB to a UE.
In the TDD, a format of a radio frame configured with 10 consecutive sub frames is also used. However, in the TDD, communication is performed using the same band in the uplink and the downlink. For this reason, as illustrated in FIG. 21, the radio frame configured with 10 consecutive sub frames #0 to #9 is shared and used such that sub frames are allocated as an uplink sub frame and a downlink sub frame (in FIG. 21, “D” represents a downlink sub frame, “U” represents an uplink sub frame, and “S” represents a special sub frame (which will be described later)).
In the past, the LTE is usually used in the FDD. However, in the FDD, there are restrictions that an uplink and a downlink are a pair, and a frequency band has to be secured. However, in the TDD, there are no such restrictions, and there is a merit that it is only necessary to secure one frequency band.
Further, let us consider from a point of view of a comparison of an uplink and a downlink in a radio frame. In the FDD, when 20 MHz is secured as each of an uplink band and a downlink band, the ratio of the uplink and the downlink is fixed to 1:1. On the other hand, in the TDD, when a band of 20 MHz is secured, as each sub frame is allocated to the uplink and the downlink as described above, it is possible to change the ratio of the uplink and the downlink in the radio frame.
In other words, since it is easy to arrange a frequency and change the ratio of the uplink and the downlink, the TDD system is expected to be increasingly used in the near further.
Meanwhile, in the TDD, it is necessary to secure a time to switch the downlink and the uplink. Specifically, when an allocation of a sub frame switches from the downlink to the uplink, it is necessary to insert “a special sub frame”. From a point of view of an eNodeB side, a downlink signal of an eNodeB is subjected to a propagation delay in space and a processing delay in a UE and thus delayed compared to a downlink position of a format until reception of the downlink signal is completed by the UE. Meanwhile, in order for an uplink signal of an UE to reach an eNodeB up to an uplink position of a format, an UE needs to start transmission of the uplink signal before the uplink position of the format. Therefore, a special sub frame inserted between a downlink sub frame and an uplink sub frame is defined by an area (a downlink pilot timeslot: DwPTS) by a delay of a downlink signal, an area (an uplink pilot timeslot: UpPTS) corresponding to a degree by which an uplink signal is transmitted early, and a gap (gap period) between the two areas. FIG. 22 illustrates an example in which a special sub frame is inserted after the sub frame #1 when switching from the downlink to the uplink is performed between the sub frame #0 and the sub frame #2 in the radio frame using the configuration illustrated in FIG. 21. As described above, the TDD has the demerit that it is necessary to insert a special sub frame when switching between the downlink and the uplink is performed (switching from the downlink to the uplink is performed).
For example, a cellular communication system in which at least one of sub frames available for uplink or downlink traffic is configured to include a portion used in uplink traffic, a portion used in downlink traffic, and a guard period portion used as a guard period scheduled between the uplink portion and the downlink portion, and at least two consecutive periods of the three portions can be changed to comply with the current necessity of a system has been proposed (for example, see Patent Document 1).
The TDD of the LTE is defined in the 3GPP Rel 8. The sub frames #0 to #9 in the radio frame are allocated for the uplink and the downlink and shared as described above. In practice, the sub frames #0 and #5 are always allocated for the downlink and used to transmit a synchronous signal from the eNodeB. FIG. 23 illustrates seven configurations 0 to 6 of the TDD defined in the LTE (TS36.211 Table 4.2-2).
Referring to FIG. 23, the sub frame #0 is fixedly allocated to the downlink in all of the configurations, the sub frame #1 is fixedly allocated to the special sub frame in all of the configurations, the sub frame #2 is fixedly allocated to the uplink in all of the configurations, and the sub frame #5 is fixedly allocated to the downlink in all of the configurations. Further, there are cases in which the sub frame #6 is allocated to the special sub frame or the downlink, and the sub frames #3, #4, #7, #8, and #9 are allocated to either of the uplink and the downlink.
In the TDD of the LTE, the seven configurations 0 to 6 illustrated in FIG. 23 are defined, but each operator is generally considered to use one of the configurations. Therefore, each operator is not considered to use different configurations in neighboring eNodeBs.
In the 3GPP plenary Meeting held in Kansas City in the U.S.A., in March, 2011, a TDD operating method of solving an interference problem using different configurations in neighboring eNodeBs has been discussed. It means that the industry in the art consequently started to move in a way of allocating different TDD configurations to neighboring eNodeB.
When neighboring eNodeBs use different TDD configurations, as can be understood from FIG. 23, links of different directions such as the uplink and the downlink are allocated at the position of at least one of the sub frames #3, #4, and #6 to #9, that is, the uplink and the downlink are mismatched.
FIG. 24 illustrates an example in which links of different directions such as the uplink and the downlink are allocated at the position of the same sub frame of neighboring eNodeBs. In FIG. 24, in a cell 1, a downlink signal is transmitted from an eNodeB to a UE, and in a cell 2, an uplink signal is transmitted from a UE to an eNodeB. It is understood that a transmission signal from the eNodeB at the time of downlink in the cell 1 serves as interference to a reception signal of the eNodeB at the time of uplink in the neighboring cell 2. Further, it can be understood that a transmission signal from the UE at the time of uplink in the cell 2 serves as interference to a reception signal of the UE at the time of downlink in the neighboring cell 1. In FIG. 24, a downlink or uplink transmission signal between the eNodeB and the UE in the same cell is indicated by a solid line, and a signal serving as interference to the neighboring cell is indicated by a dotted line.
FIG. 25 illustrates an example in which different TDD configurations are used in relative large areas. For example, such configuration switching occurs in the boundary between Chiba Prefecture and Tokyo Metropolitan. In FIG. 25, the configuration 0 is used in the left area, and the configuration 1 is used in the right area. Referring back to FIG. 23, when the area using the configuration 0 is adjacent to the area using the configuration 1, the uplink and the downlink are mismatched at the positions of the sub frames #4 and #9.
FIG. 25 illustrates an example in which the sub frame #4 is allocated for the uplink (UP) in the left area using the configuration 0 but allocated for the downlink (DN) in the right area using the configuration 1. When different TDD configurations are used in relative large areas, a boundary surface in which the uplink and the downlink are mismatched extends across a relative broad area as indicated by a thick line in FIG. 25. Further, there occurs a problem in that along the mismatch boundary surface, a transmission signal from an eNodeB at the time of downlink serves as interference to a reception signal of a neighboring eNodeB at the time of uplink, and a transmission signal from a UE at the time of uplink serves as interference to a reception signal of a UE at the time of downlink in a neighboring cell.
FIG. 26 illustrates an example in which cells using different TDD configurations are located a spot-like manner. In FIG. 26, in an area using the configuration 1, only a cell indicated by a thick line is assumed to use the configuration 0. When the area using the configuration 0 is adjacent to the area using the configuration 1, the uplink and the downlink are mismatched at the positions of the sub frames #4 and #9 (same as above). In FIG. 26, a spot-like cell that uses the configuration 0 and is allocated the uplink (UP) for the sub frame #4 is surrounded by cells that use the configuration 1 and are allocated the downlink (DN) for the sub frame #4. In this case, a problem in which the uplink and the downlink are mismatched occurs locally.